kj/async-unix-test: fix -Wmissing-field-initializers in UrgentObserver

Warned by gcc 5.3.1 and clang 3.7.0

c++/src/kj/async-unix-test.c++

@@ -470,7 +470,7 @@ TEST(AsyncUnixTest, UrgentObserver) {
   // Spawn a TCP server
   KJ_SYSCALL(tmpFd = socket(AF_INET, SOCK_STREAM, 0));
   kj::AutoCloseFd serverFd(tmpFd);
-  sockaddr_in saddr = {0};
+  sockaddr_in saddr = {};
   saddr.sin_family = AF_INET;
   saddr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
   KJ_SYSCALL(bind(serverFd, reinterpret_cast<sockaddr*>(&saddr), sizeof(saddr)));